1. Field of the Invention
This invention relates to tools used in die-casting aluminum alloys and a method of making such tools which can include the dies, the fill chamber or shot sleeve, and other components of the apparatus and particularly those exposed to the molten aluminum. It is directed to such tools, and a method of making such tools, with a single or multi-layer coating comprising a material which resists dissolution and erosion by molten aluminum alloys, particularly low iron aluminum alloys.
2. Background Information
Tooling for die-casting molten aluminum is commonly fabricated from tool steel, such as H13 alloy steel. As molten aluminum is highly reactive, the surface of the H13 steel tooling is generally treated to resist the dissolution and erosion of the steel. Typically, the steel tool is nitrided such as by ion (plasma), salt bath, or gas treatment. This nitride treatment is adequate for protecting the steel tooling from aluminum alloys which typically contain 0.6% by weight or more of iron. There is interest, however, in reducing the iron content of some aluminum alloys below 0.6% and preferably below 0.3% by weight to improve ductility, strength and treatability of the casting. This reduction in iron content of the aluminum alloy increases its reactivity with the steel tooling resulting in increased dissolution, soldering, and erosion, particularly in areas exposed to molten aluminum at high velocities. This dissolution and erosive wear limits the life of the die-casting tooling when it occurs in a critical area.
It has been suggested in Japanese Kokai patent announcement 56-111560, dated Sep. 3, 1981, that copper die-casting tooling can be protected from aluminum by coating the tooling with silicon carbide which has low wettability with aluminum. As the silicon carbide and copper have substantially different coefficients of thermal expansion, a base layer of titanium nitride or titanium carbide, which have coefficients of thermal expansion intermediate to those of copper and silicon carbide, is applied to the copper tooling first. The silicon carbide, and titanium carbide or nitride are applied by a physical vapor deposition process rather than a chemical vapor deposition process, as the latter requires a reaction temperature that would significantly soften or even melt copper.
It has also been suggested in Japanese patent publication 55-41298 that dies for casting high melting point metals such as iron, can be made resistant to corrosion by using tungsten or molybdenum alloys as the base material, and coating the base material with at least one species of titanium nitride, titanium carbide, alumina, zirconia and silicon nitride deposited by chemical vapor deposition. The tungsten and molybdenum alloys can withstand the high temperature, about 1000.degree. C. required for conventional chemical vapor deposition, without significantly changing their mechanical properties. However, as mentioned above, such high reaction temperatures would soften/distort copper dies. They also would distort steel dies and seriously affect their mechanical properties.
At least one investigator has considered coating steel dies for casting aluminum with CrN and TiN by physical vapor deposition or with TiN, TiC, Ti(CN), Ti(CN)--TiN, TiC--Ti(CN)--TiN, or W2C by chemical vapor deposition as reported in an article entitled: "Extension to Service Life of Dies for Aluminum Die-Casting; by Masahiko Hihara, Yamanashi Industrial Technological College, Kinzoku; Vol. 58, No. 8 34-41 (1988). The CrN coating applied by a physical vapor deposition process was considered unsatisfactory as the coating exfoliated and was not much better than the bare steel in resisting corrosion by the molten aluminum. It appears that the CrN coating was about 3 microns thick.